The invention relates to a method for determining the structure of a transistor which comprises at least a first layer, a second layer and a fourth layer, wherein the fourth layer is arranged on the second layer and the second layer is arranged on the first layer, the first layer comprises GaN, the second layer comprises AlxGa1-xN and the fourth layer comprises a metal or an alloy. The invention also relates to a semiconductor element which comprises a transistor.
U.S. Pat. No. 7,224,004 B2 discloses a transistor of the type cited at the outset. This transistor has at least a first and a third layer which comprise GaN. Arranged in between these is a second layer which comprises AlGaN. On account of the various lattice constants, mechanical tensions are produced within the semiconductor structure which cause piezoelectric polarization. This produces band bending at the interface between the first and second layers, and a two-dimensional electron gas is produced at said band bending. This can produce a zone of increased electrical conductivity within the transistor, without having to introduce dopants into the material. By applying an electrical voltage to a fourth layer, comprising a metal, that has been applied to the third layer, it is possible to influence the charge carrier density during operation of the transistor.
Transistors of the type cited at the outset are used particularly as power amplifiers for radio-frequency signals, for example signals at a frequency of more than 0.9 GHz. Such transistors can be fabricated by means of metalorganic vapor phase epitaxy or molecular beam epitaxy, for example.
The charge carrier density of the two-dimensional electron gas (2-DEG) influences the layer resistance within the 2-DEG and also the threshold value of the gate voltage of a transistor comprising said semiconductor structure, from which the flow of current within the 2-DEG starts. In addition, the threshold voltage is influenced by the surface potential of the semiconductor structure, the thickness thereof and the magnitude of the band bending which occurs at the interface. O. Ambacher et al.: Pyroelectric properties of AlGaN/GaN hetero- an quantum-well structures, J. Phys.: Condens. Matter 14 (2002) 3399 discloses an approximation formula which can be used to model the charge carrier concentration within the two-dimensional electron gas as a function of the aluminum content of the second layer.
The technical application of the transistors cited at the outset requires the provision of such a transistor having a prescribable threshold voltage Uth and having a prescribable layer resistance. It is therefore an object of the present invention to specify a method for determining the structure of a transistor which has a prescribable threshold voltage Uth and/or a prescribable charge carrier density ns.